1. Field of the Invention
This invention relates to an attenuating device, and more particularly, to a device which utilizes a YIG material to provide frequency selective attenuation of microwave signals above a preselected threshold power level.
2. Description of the Prior Art
Frequency selective limiting (FSL) or attenuating devices which utilize a yttrium-iron-garnet (YIG) material have the property of being able to attenuate higher power level signals while simultaneously allowing lower power level signals separated by only a small frequency offset from the higher level signals to pass with relatively low loss. YIG-based FSL's are capable of limiting or attenuating across more than an octave bandwidth in the 2-8 GHz range. Higher power level (above-threshold) signals within this selectivity bandwidth will be attenuated without requiring tuning of the FSL. Lower power level (below-threshold) signals separated from the higher power level signals by more than a few spinwave linewidths will pass through the FSL without experiencing any greater loss than if the higher power level signals were not present. For an attenuating device based on YIG, this selectivity bandwidth is on the order of between 20-50 MHz.
YIG-based FSL's have many applications in microwave signal systems, one such application being illustrated in FIG. 1. As seen in FIG. 1, the microwave signal system 10 includes an antenna 12 for collecting and passing microwave signals to a broadband-type receiver 14. The microwave signal system 10 also includes a YIG-based FSL device generally designated by the numeral 16 interposed between antenna 12 and broadband-type receiver 14.
FSL 16 is utilized as illustrated in FIG. 1 to increase the dynamic range over which microwave signals collected by antenna 12 can be measured by broadband-type receiver 14. These microwave signals measured by receiver 14 may thereafter be supplied to processor 17, or to any other suitable device. Since known receivers such as broadband-type receiver 14 generally have a dynamic range of approximately 35 dB, and signals of interest arriving at antenna 12 may have a dynamic range of, for example, 85 dB, it is seen that a power mismatch is created within system 10. This mismatch in dynamic ranges between signals arriving at antenna 12 and broadband-type receiver 14 may be corrected by utilizing an attenuation device such as FSL device 16. If, as previously stated, the dynamic range of signals arriving at antenna 12 is 85 dB and the dynamic range of broadband-type receiver 14 is 35 dB, then FSL 16 will be designed to provide a dynamic range of approximately 50 dB. In this manner, the total dynamic range of FSL device 16 and broadband-type receiver 14 is matched with the dynamic range of signals at antenna 12. FSL device 16 is designed to provide that the ratio of power out to power in (P.sub.out /P.sub.in) below a predetermined threshold value of P.sub.in is substantially linear. As the value of P.sub.in seen by FSL device 16 increases above the predetermined threshold value of P.sub.in, the ratio of P.sub.out /P.sub.in becomes compressed. The compression of this ratio indicates that, for substantial increases in the value of P.sub.in above the threshold value of P.sub.in, the corresponding value of P.sub.out is relatively small. For example, FSL 16 may be designed to provide that, for a 50 db increase in P.sub.in above the threshold power level of P.sub.in, the corresponding increase in the level of P.sub.out above the level of P.sub.out at threshold P.sub.in would be approximately 5 db. Stated in another manner, FSL 16 operates to attenuate an above-threshold, input microwave signal having a dynamic range of 50 db to provide an output signal having a dynamic range of 5 db.
Although FSL device 16 described in FIG. 1 provides satisfactory above-threshold signal attenuation, the construction of FSL device 16 results in a relatively low level of above-threshold attenuation per unit length of the device. This requires meandering of the signal-carrying conductor which forms a part of the device over a relatively extended distance in order to develop adequate limiting.
YIG-based frequency selective limiting devices have heretofore been constructed using: single crystal YIG bars arrayed along a sidewall of a rectangular waveguide, YIG spheres in stripline and coaxial structures, and thin YIG plates or slabs in microstrip structures.
With each of the above-described YIG-based FSL's it has been found that the amount of level of signal attenuation capable of being achieved at a given power level above a threshold power level is proportional to the volume of ferrite or YIG material coupled to the RF field generated as the signal is passed through the FSL. In these known YIG-based FSL's, the configuration of the YIG material and the positioning of the YIG material relative to the signal-carrying conductor results in the majority of the RF field lines generated by a microwave signal flowing through the conductor to pass through regions not filled with YIG material. These generated RF field lines do not interact with the YIG material and as a result contribute nothing to the attenuation of the above-threshold microwave signal.
Therefore, there is a need for an improved YIG-based frequency selective limiting device in which the YIG material is arranged to provide maximum interaction with RF field lines generated by a microwave signal passed through the signal-carrying conductor of the device to maximize the limiting or attenuation of an above-threshold signal for a given length of conductor. The device must be capable of attenuating microwave signals having a power level above a preselected threshold power level while allowing microwave signals below the threshold power level to pass substantially undisturbed.
Accordingly, the principal object of the present invention is to provide an improved ferrite-based frequency selective limiting or attenuating device capable of providing a greater degree of above-threshold signal attenuation than ferrite-based attenuators presently utilized.
It is a further object of the present invention to provide a frequency selective limiting device which includes a plurality of individual ferrite-based limiting or attenuating units each interposed between a pair of magnetic strips which provide a DC bias field for the ferrite.
It is yet another object of the present invention to provide a method for making an individual ferrite-based limiting or attenuating unit which forms a part of an improved multi-unit frequency selective limiting device.
These and other objects of the present invention will be more completely disclosed and described in the following description, the accompanying drawings and the appended claims.